Paul wonders aloud why they must always fit everything around Matt. (thearchers.co.uk)
What does the highlighted part mean?
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It means that no matter what they are doing, they modify their schedule or their plans in order to accommodate Matt's needs. For example, if they want to meet up at a party but Matt cannot get there on his own, they will make sure to pick him up. If they want to go to a movie in the afternoon but Matt works until dinnertime, they will go in the evening instead, when Matt is able to attend. Whatever they do, they make sure that it fits into Matt's schedule and takes care of his requirements.
If you consider Matt's set of needs as a rigid, inflexible object, then their plans are always being modified and re-shaped to fit comfortably around those needs, so there is never a clash or an overlap.
To make everything which is around Matt,comfortable,suitable and according to his taste and desire for him. I guess! Because the context in the link tells us:
Lilian is spotted by one of Matt’s friends, Andrew Eagleton. She quickly comes up with a cover story, which Paul thinks Andrew believes. Paul gets increasingly annoyed by Lilian’s paranoia and is frustrated when she wants to leave early. Exasperated, Paul wonders aloud why they must always fit everything around Matt. Lilian promises to try and make things up to him, but she just can’t stay there anymore.
It seems there is a near relationship between Lilian and Paul so she is trying to hide it as you can find the matter when they decide to make a fake story to a common friend who has seen Lilian with Paul in that place/situation and may talk about it to the others or Matt. It seems Matt is important for Lilian and she doesn't want to bother him by this story and tries to arrange everything according to his desires opposite of Paul's feelings who is angry because of it and doesn't believe in Lilian's promises to make things up to him but for Matt because she is coming back and can't stay there anymore!